Clock circuits are used in many analog ICs to provide reference clock signals. One particular type of clock circuit is a real time clock (RTC). An RTC is used to provide a reference clock, and can also be used to provide real time clock functions (i.e., date and time functions). An RTC typically includes an analog portion and a digital portion. The analog portion typically includes an oscillator and buffer circuitry, while the digital portion typically includes digital counters for implementing the real time functions.
Typically, the oscillator of the analog portion of a real time clock is a crystal oscillator that uses load capacitors to generate a desired oscillation frequency. The value of the load capacitors may vary, depending on a crystal chosen for use in a system. Accordingly, load capacitors are typically implemented using off-chip capacitors, increasing component counts, as well as consuming valuable board area.
Because an RTC is used to provide real time clock functions, it must always be supplied power, even when remaining portions of the IC (and even the system in which it is contained) is powered off. Accordingly, an RTC needs to operate at very low power consumption levels and it typically runs on a backup power supply, such as a coin cell battery or the like. To conserve power in this backup power supply, some systems switch to provide power to the RTC from a main power supply when available. However, such switching schemes are complex and inefficient, and consume additional power.
A need thus exists to provide more flexibility in the use of load capacitors, as well as to reduce power consumed by a back up power supply.